In view of the vegetation reduction caused by the continuous construction of oil and gas pipelines, the pipelines have been designed to be laid in one ditch to reduce land occupation. However, owing to the small spacing between the pipelines, the fault correlation between pipelines has been proven to increase the potential hazard of adjacent pipelines and routing environments. The neglect of failure correlation in existing risk assessment methods leads to inaccurate results, which will lead to errors in maintenance decisions. Therefore, this paper proposed a risk assessment system for pipelines using this laying method. In the risk assessment, pipelines laid in one ditch (PLOD) were regarded as a series system relative to the routing environment. Therefore, the functional relationship between the total risk of the pipeline system and the risk of each pipeline was obtained by combining the engineering system reliability theory with the mathematical induction method. In addition, fuzzy set theory combined with fault tree analysis was used to calculate the failure probability of each pipeline in the system. Event tree analysis was used to sort out all the possible consequences of pipeline failure, and then the consequences were unified into monetary units to evaluate the severity of failure consequences. Finally, the two parts were merged into a bow-tie diagram to realize the risk management and control of the pipeline. Meanwhile, risk acceptance criteria were formulated to analyze risks and to guide pipeline maintenance. This system provides a complete risk assessment system for the pipeline system laid in one ditch, including the methods of risk identification, risk assessment, and risk analysis, which are of great significance to ensure the safety of the pipeline and the surrounding environment using this laying method.